Positive active material for alkaline battery and electrode using the same

ABSTRACT

A nickel hydroxide positive active material for an alkaline battery contains nickel hydroxide powder having a nickel valence of greater than 2; and a cobalt compound having a cobalt valence of greater than 2, which is formed on the surface of said nickel hydroxide powder. For example, the surface of nickel oxyhydroxide powder is covered by cobalt oxyhydroxide layer. This positive active material is used as a starting material to produce an electrode by retaining it in a three-dimensional porous material.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a nickel hydroxide positive activematerial for an alkaline battery and a nickel hydroxide electrode usingthe nickel hydroxide active material.

2. Description of the Related Art

A nickel hydroxide electrode comprising an active material as a compoundmainly composed of nickel hydroxide has been used as a positiveelectrode for an alkaline battery such as a nickel-cadmium storagebattery and a nickel-metal hydride storage battery. Such an alkalinestorage battery is used as a power supply for small-sized portableelectronic devices. In order to improve these electronic devices,alkaline storage batteries having an enhanced capacity have beendesired. To this end, it is necessary that the nickel hydroxideelectrode, which governs the discharge capacity of alkaline storagebattery, have an enhanced high energy density.

As a nickel hydroxide electrode, there has heretofore been often used asintered electrode. The sintered electrode is prepared by sintering anickel powder on a porous punching metal to obtain a substrate, and thenimpregnating the substrate with an nickel hydroxide active material.However, such a sintered nickel hydroxide electrode is disadvantageousin that the substrate exhibits porosity as small as about 80%, and thuscan be hardly impregnated with a large amount of an active material.Therefore, such a substrate is unfavorable for the enhancement of theenergy density of nickel hydroxide electrode.

On the other hand, a non-sintered nickel hydroxide electrode obtained byretaining a nickel hydroxide active material in a three-dimensionalporous metal material such as foamed nickel or fibrous nickel as asubstrate exhibits a substrate porosity as high as not less than 95%.Thus, it is favorable for the enhancement of the energy density ofnickel hydroxide electrode as compared with the foregoing sinterednickel hydroxide electrode. Under these circumstances, studies ofenhancement of capacity of alkaline storage batteries have been mademainly using non-sintered nickel hydroxide electrode.

The non-sintered nickel hydroxide electrode exhibits a low electricalconductivity of nickel hydroxide. Accordingly, it is necessary toincorporate a cobalt compound such as cobalt hydroxide, cobalt monoxideand cobalt suboxide or metallic cobalt as an electrically-conductingagent in the nickel hydroxide electrode or coat the surface ofparticulate nickel hydroxide with such a cobalt compound or metalliccompound as disclosed in JP-A-62-117267 (The term “JP-A” as used hereinmeans an “Unexamined Japanese Patent Publication”).

It is thought that the cobalt compound thus added is converted to cobaltoxyhydroxide having a high electrical conductivity by electrochemicaloxidation during the first charging for formation. Then, it forms anelectrically conductive network in the electrode to function as aneffective electrically conducting agent in the nickel hydroxideelectrode. The coating of the surface of particulate nickel hydroxidemakes it possible to raise the area of contact of the cobalt compoundwith nickel hydroxide and hence add to the effect of enhancing thepercent utilization of active material as compared with the singleaddition of the cobalt compound.

Nickel hydroxide, which acts as an active material for nickel hydroxideelectrode in alkaline storage batteries, is oxidized during charging toform nickel oxyhydroxide. Further, it is reduced to nickel hydroxideduring discharging. Nickel hydroxide is a material having a lowerelectrical conductivity than nickel oxyhydroxide. Thus, nickel hydroxideis disadvantageous in that it causes a drop of charge efficiency at theinitial stage of charging, particularly during the first charging forformation.

In order to solve these problems, an approach has been proposedinvolving the uniform addition of nickel oxyhydroxide, which is amaterial having a higher electrical conductivity than nickel hydroxide,to a nickel hydroxide electrode material before the preparation of anickel hydroxide electrode as described in JP-A-2-262245 andJP-A-2-234356.

However, it was found that the addition of nickel oxyhydroxide to anickel hydroxide electrode having the conventional cobalt compoundhaving a cobalt valence of 2 or less such as cobalt hydroxide and cobaltmonoxide or metallic cobalt incorporated therein gives a smallerdischarge capacity than calculated.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved nickelhydroxide active material for an alkaline battery and a nickel hydroxideelectrode using the improved nickel hydroxide active material.

The inventors made studies of the reason for the above phenomenon. As aresult, it was concluded that the cobalt compound having a cobaltvalence of not more than 2, even if it partly has a cobalt valence ofgreater than 2 but lower than the valence of nickel hydroxide having avalence of greater than 2, partly undergoes oxidation by nickeloxyhydroxide, which has a high oxidizing powder, to form a high ordercobalt compound that causes the maldistribution of cobalt compound inthe nickel hydroxide electrode, making it impossible to sufficientlyform a network of electrical conductivity of cobalt oxyhydroxide, whichis considered required to maintain the electrical conductivity of thenickel hydroxide electrode. It was found on the basis of this conclusionthat the previous formation of cobalt oxyhydroxide on the surface ofparticulate nickel hydroxide gives solution to the foregoing problems.The inventors further found a method for enhancing the dischargecapacity of alkaline battery in the course of their studies.

According to the present invention, a nickel hydroxide positive activematerial for an alkaline battery comprises: nickel hydroxide powderhaving a nickel valence of greater than 2; and a cobalt compound havinga cobalt valence of greater than 2, which is formed on the surface ofsaid nickel hydroxide powder.

According to the present invention, an electrode is composed of athree-dimensional porous material retaining the above nickel hydroxidepositive active material therein.

In accordance with the present invention, the percent utilization ofactive material in the nickel hydroxide electrode can be enhanced,making it possible to provide the alkaline battery with an enhancedcapacity.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a graph illustrating the discharge characteristics of anembodiment of the nickel-cadmium battery according to the presentinvention; and

FIG. 2 is a graph illustrating the discharge characteristics of anotherembodiment of the nickel-metal hydride battery according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Detailed description of the present invention will be described asfollows.

A nickel hydroxide positive active material for an alkaline batteryaccording to the present invention is mainly composed of nickelhydroxide powder having a nickel valence of greater than 2. A cobaltcompound having a cobalt valence of greater than 2 is formed on thesurface of the nickel hydroxide powder.

The average particle size of the nickel hydroxide powder in the presentinvention is in the range of 7 to 30 μm, preferably about 10 μm. Theaverage particle size includes the thickness of the cobalt compoundformed thereon.

A nickel hydroxide electrode according to the present invention iscomposed of a three-dimensional porous material retaining the abovenickel hydroxide positive active material therein.

The nickel hydroxide powder according to the present invention is mainlycomposed of nickel hydroxide having a nickel valence of greater than 2.If the nickel valence of the nickel hydroxide powder exceeds 3, γ-nickeloxyhydroxide, which is considered to have an adverse effect on thecharge-discharge cycle life of alkaline storage battery, is produced.Therefore, the nickel valence of nickel hydroxide to be used as a maincomponent is preferably not more than 3.

The cobalt compound formed on the surface of the nickel hydroxide powderhaving a nickel valence of greater than 2 is preferably arrangedcovering the nickel hydroxide powder in a layer. If the cobalt valenceof the cobalt compound is smaller than the nickel valence of the nickelhydroxide powder, the cobalt compound can be easily oxidized due to thestrong oxidizing power of the nickel hydroxide powder, making itdifficult to maintain its electrical conductivity. Therefore, the cobaltvalence of the cobalt compound is preferably equal to or greater thanthe nickel valence of the nickel hydroxide powder. In other words, thiscobalt compound is one of the materials responsible for the electricalconductivity of the nickel hydroxide powder which acts as an activematerial. Thus, this cobalt compound preferably has a high electricalconductivity. It is further preferred that the average nickel valence ofthe nickel hydroxide powder do not exceed the average cobalt valence ofthe cobalt compound having a cobalt valence of greater than 2 by whichthe nickel hydroxide powder is covered.

The foregoing three-dimensional porous material is preferably athree-dimensional porous metallic material such as foamed nickel andfibrous nickel.

Further, the nickel hydroxide electrode according to the presentinvention preferably is made of an admixture of a first nickel hydroxidepowder having a nickel valence of not greater than 2 with a cobaltcompound having a cobalt valence of greater than 2 formed on the surfacethereof and a second nickel hydroxide powder having a nickel valence ofgreater than 2 with a cobalt compound having a cobalt valence of greaterthan 2 formed on the surface thereof, and a three-dimensional porousmaterial retaining the admixture. In particular, the average nickelvalence of the first and second nickel hydroxide powder of the admixtureretained in the three-dimensional porous material is preferably not lessthan 2.07.

This is because the nickel hydroxide having a valence of greater than 2exerts a reduced effect of enhancing the electrical conductivity whenthe average nickel valence of the entire nickel hydroxide activematerial containing the nickel hydroxide does not exceed 2.07.

In the nickel hydroxide electrode according to the present invention,the average nickel valence of the nickel hydroxide powder having anickel valence of greater than 2 contained in the electrode preferablydoes not exceed the average cobalt valence of the cobalt compound havinga cobalt valence of greater than 2 by which the nickel hydroxide powderis covered.

The nickel hydroxide active material which is a main component of thepowder of the present invention has a nickel valence of greater than 2.Accordingly, the powder according to the present invention exhibits ahigher electrical conductivity than the conventional nickel hydroxidepowders. Further, since the cobalt compound layer having a cobaltvalence of greater than 2 formed on the surface of the nickel hydroxidepowder also exhibits a high electrical conductivity, the nickelhydroxide powder of the present invention can be used to prepare anickel hydroxide electrode having a high electrical conductivity thatcan enhance the percent utilization of the nickel hydroxide electrode inan alkaline battery.

The nickel hydroxide active material for an alkaline battery accordingto the present invention can be used as a positive active material foran alkaline battery, particularly an alkaline storage battery. Thenickel hydroxide powder for alkaline battery according to the presentinvention may be singly used as an active material. Alternatively, thenickel hydroxide powder for an alkaline battery according to the presentinvention may be used in admixture with an active material made of apowder mainly composed of nickel hydroxide as prepared by forming cobaltoxyhydroxide on the surface of particulate nickel hydroxide, having acobalt compound having a cobalt valence of greater than 2 formed on thesurface of the powder. The admixture may further contains other activematerials or additives incorporated therein. Particularly, the nickelhydroxide powder is used in admixture with a nickel hydroxide powderhaving a nickel valence of not greater than 2 with a cobalt compoundhaving a cobalt valence of greater than 2 formed on the surface thereof,it is easy to control the nickel valence in nickel hydroxide activematerial in the positive electrode.

Further, the active material of the present invention can be used as astarting material of electrode. Preferably, the active material of thepresent invention is retained on a three-dimensional porous support toprepare an electrode.

EXAMPLES

The present invention will be further described in the followingexamples and comparative examples.

Example 1

A nickel hydroxide powder was dispersed in purified water in an amountof three times that of the powder by weight. The aqueous dispersion wasthen adjusted to weak alkaline as pH 11 with a 23 wt-% aqueous solutionof sodium hydroxide. To the aqueous dispersion was then added dropwisean aqueous solution of cobalt sulfate with stirring in an atmosphere ofinert gas to prepare an nickel hydroxide active material made of anickel hydroxide powder covered by 10 wt-% cobalt hydroxide on thesurface thereof. Subsequently, the alkaline suspension containing theactive material was withdrawn in the air. The alkaline suspension thuswithdrawn was heated to a temperature of 80° C. with stirring for 3hours, washed, and then dried to obtain nickel hydroxide covered by acobalt compound having a valence of greater than 2. The sample was thensubjected to chemical analysis. As a result, the cobalt compound coatinglayer was cobalt oxyhydroxide having a cobalt valence of 3.0. The nickelvalence of the nickel hydroxide powder was 2.0.

330 g (nickel hydroxide content: about 3.24 mols) of the foregoingnickel hydroxide covered by cobalt oxyhydroxide was then dispersed in5000 cc of a 5 wt-% aqueous solution of sodium hydroxide at roomtemperature. To the dispersion was then added 231 g (about 0.98 mols) ofsodium peroxodisulfate as an oxidizer with stirring to cause reactionaccording to the following reaction formula (1). After 10 hours ofstirring, the reaction product was washed and dried to obtain an activematerial powder A.

2Ni(OH)₂+S₂O₈ ²⁻+2OH⁻→2NiOOH+2SO₄ ²⁻+2H₂O  (1)

The active material powder A was then subjected to chemical analysis. Asa result, the active material powder A was found to have a cobaltvalence of 3.0 and a nickel valence of 2.5. The active material powder Awas also subjected to X-ray diffractometry. As a result, the activematerial powder A was confirmed to show diffraction peaks correspondingto cobalt oxyhydroxide, nickel hydroxide and β-nickel oxyhydroxide.

100 parts by weight of the active material powder A thus obtained, 1part by weight of carboxymethyl cellulose and purified water were thenused to prepare a slurry sample. A foamed nickel substrate wasimpregnated with the slurry sample, pressed, and then dried to obtain anickel hydroxide electrode A made of the active material powder A of thepresent invention.

As a negative electrode, there was used a known negative cadmium plateprepared by applying a slurry sample mainly composed of 70 parts byweight of a cadmium oxide powder and 30 parts by weight of a metalliccadmium powder to a punching metal, drying the punching metal, and thenpressing the punching metal.

3 sheets of the positive electrode plates thus obtained, 4 sheets of thenegative electrode plates thus obtained and a hydrophilically-treatedpolyolefin separator were then used to form an element. The element thusformed was inserted into a battery can filled with an electrolytesolution mainly composed of an aqueous solution of potassium hydroxide.To the battery can was then welded a cover having a safety valveincorporated therein to obtain a battery A of the present inventionincluding the active material powder A of the present invention.

Comparative Example 1

100 parts by weight of nickel hydroxide covered by cobalt oxyhydroxideobtained as a precursor of the active material powder A of the presentinvention, 1 part by weight of carboxymethyl cellulose and purifiedwater were used to prepare a slurry sample. A foamed nickel substratewas impregnated with the slurry sample, pressed, and then dried toprepare a nickel hydroxide electrode B. A comparative battery B was thenobtained in the same manner as in Example 1 except that the nickelhydroxide electrode B thus prepared was used.

Comparative Example 2

200 g (about 2.16 mols) of a nickel hydroxide powder was dispersed in 2l of a 5 wt-% aqueous solution of sodium hydroxide. To the dispersionwas then added 309 g (about 1.30 mols) of sodium peroxodisulfate as anoxidizer with stirring to cause reaction according to the foregoingreaction formula (1). After 10 hours of stirring, the reaction productwas washed and dried to obtain an active material powder C.

As a result of chemical analysis, the active material powder C thusobtained was found to have a nickel valence of 3.0. The results of X-raydiffractometry showed that the active material powder C composed ofβ-nickel oxyhydroxide.

The active material powder C thus obtained and a nickel hydroxide powdercovered by 10 wt-% cobalt oxyhydroxide were then dry-mixed at a weightratio of 45:50. 100 parts by weight of the mixture powder, 1 part byweight of carboxymethyl cellulose and purified water were used toprepare a slurry sample. A foamed nickel substrate was impregnated withthe slurry sample, pressed, and then dried to obtain a nickel hydroxideelectrode C composed of the active material powder C. A comparativebattery C was then prepared in the same manner as in Example 1 exceptthat the nickel hydroxide electrode C was used.

The positive electrodes of all these batteries have the theoreticalcapacity of 1,000 mAh. These batteries were prepared with the capacityof the negative electrode being excessive such that the positiveelectrode can act as a capacity-limiting electrode during both chargingand discharging.

These batteries were each subjected to 10 cycles of 12 hour charging at100 mA and discharging to 0.8 V at 100 mA at 25° C., and then measuredthe discharge capacity under the following conditions:

Charging . . . 120% (6 hours) at 200 mA

Discharging . . . To 0.8 V at 200 mA

Temperature . . . 25° C.

The results of test are shown in FIG. 1. The battery A of the presentinvention using an active material mainly composed of nickel hydroxidehaving a nickel valence of 2.5 covered by cobalt oxyhydroxide exhibiteda discharge capacity as great as about 1,030 mAh. On the other hand, thecomparative battery B using nickel hydroxide covered by cobaltoxyhydroxide and the comparative battery C using a nickel oxyhydroxidepowder and a nickel hydroxide powder covered by cobalt oxyhydroxideexhibited a discharge capacity of about 980 mAh and about 1,000 mAh,respectively.

The batteries A and C using nickel oxyhydroxide incorporated in itspositive electrode exhibited a higher discharge capacity than thecomparative battery B, which is free of nickel oxyhydroxide. This ispresumably attributed to the fact that the incorporation of nickeloxyhydroxide in the positive electrode causes the electricalconductivity of the active material itself to be raised, enhancing thepercent utilization of the active material.

The reason why the battery A of the present invention, which uses anactive material mainly composed of nickel hydroxide having a nickelvalence of 2.5 covered by cobalt oxyhydroxide exhibits a greaterdischarge capacity than the comparative battery C, which uses nickeloxyhydroxide and nickel hydroxide covered by cobalt oxyhydroxide, ispresumably because the battery A of the present invention uses a nickelhydroxide powder uniformly covered by cobalt oxyhydroxide, which canmaintain its electrical conductivity even at the last stage ofdischarging.

The example has been described with reference to a nickel-cadmiumstorage battery having a cadmium electrode as a negative electrode.However, similar effects can be exerted also with other alkaline storagebatteries using a zinc electrode or metal hydride electrode as anegative electrode.

The nickel hydroxide active material of the present invention can exertsimilar effects even when it contains at least one element selected fromthe group consisting of cobalt, manganese, zinc, cadmium and calciumsolid-dissolved therein besides the single composition particles made ofnickel compound alone.

Example 2

A nickel hydroxide powder was dispersed in purified water in an amountof about three times that of the powder by weight. 23-wt % potassiumhydroxide was added to the dispersion to adjust its pH as 11. While thisdispersion liquid was stirred, an aqueous solution of cobalt sulfate wasdropped in an inert gas atmosphere. Then, nickel hydroxide activematerial was produced which the surface of nickel hydroxide powder wascovered by 10 wt-% cobalt hydroxide with respect to nickel hydroxide.Then, the alkaline suspension containing the active material waswithdrawn to the air, and was stirred for 3 hours at 80° C. Thereafter,it was washed, and then dried to obtain a nickel hydroxide powdercovered by a cobalt compound having a cobalt valence of greater than 2.The sample thus obtained was then subjected to chemical analysis. As aresult, the valence of the cobalt compound coating layer was found to be3.0. The nickel hydroxide powder was found to have a valence of 2.0.

Subsequently, the nickel hydroxide powder covered by a cobalt compoundhaving a valence of greater than 2 and a potassium peroxodisulfatepowder were dispersed in an aqueous solution of 26 wt-% potassiumhydroxide in an amount of about 20 times that of the nickel hydroxidepowder covered by a cobalt compound having a valence of greater than 2at a molar ratio of 2:1.4 to cause reaction according to the followingreaction formula (1). The reaction solution was then stirred at roomtemperature for 10 hours. After reaction, the dispersion was filtered toobtain a black powder. The black powder thus obtained was washed withpurified water until the pH value of the wash water reached 7, and thendried to obtain a nickel oxyhydroxide powder covered by a cobaltcompound having a valence of greater than 2. The nickel oxyhydroxidepowder thus obtained also was subjected to chemical analysis in the samemanner as mentioned above. As a result, the nickel oxyhydroxide powderwas found to have a nickel valence of 3.0 and a cobalt valence of 3.0.In the present example, the cobalt coating layer has exhibited a valenceof 3 before oxidation. Therefore, it is thought that the cobalt coatinglayer didn't undergo oxidation by potassium peroxodisulfate.

2Ni(OH)₂+S₂O₈ ²⁻+2OH⁻→2NiOOH+2SO₄ ²⁻+2H₂O  (1)

85 parts by weight of the foregoing nickel hydroxide powder covered by acobalt compound having a valence of 3, 15 parts by weight of a nickeloxyhydroxide powder covered by a cobalt compound having a valence of 3and 2 parts by weight of a carboxymethyl cellulose powder were thenadded to purified water to prepare a slurry sample. A foamed nickelsubstrate was impregnated with the slurry sample, pressed, and thendried to obtain a nickel hydroxide electrode D of the present invention.The nickel hydroxide electrode D thus obtained was then examined foraverage nickel valence. As a result, the nickel hydroxide electrode Dwas found to have an average nickel valence of 2.15.

As a negative electrode there was used an electrode prepared by applyinga slurry sample mainly composed of a known AB5 type hydrogen-occludingalloy to punching metal, drying the coated material, and then pressingthe punching metal.

3 sheets of the positive electrode plates thus obtained, 4 sheets of thenegative electrode plates thus obtained and a hydrophilically-treatedpolyolefin separator were then used to form an element. The element thusformed was inserted into a battery can filled with an electrolytesolution mainly composed of an aqueous solution of potassium hydroxide.To the battery can was then welded a cover having a safety valveincorporated therein to obtain a battery D of the present invention.

Comparative Example 3

A nickel hydroxide powder was dispersed in purified water in an amountof about three times that of the powder by weight. 23 -wt% potassiumhydroxide was added to the dispersion to adjust its pH as 11. While thisdispersion liquid was stirred, an aqueous solution of cobalt sulfate wasdropped in an inert gas atmosphere. Then, nickel hydroxide activematerial was produced which the surface of nickel hydroxide powder wascovered with 11.3 wt-% cobalt hydroxide with respect to nickelhydroxide. Then, the alkalinene suspension containing the activematerial was withdrawn to the air, and was stirred for 3 hours at 80° C.Thereafter, it was washed, and then dried to obtain a nickel hydroxidepowder covered by a cobalt compound having a cobalt valence of greaterthan 2. The sample thus obtained was then subjected to chemicalanalysis. As a result, the valence of the cobalt compound coating layerwas found to be 3.0. The nickel hydroxide powder was found to have avalence of 2.0.

A nickel hydroxide electrode E was then prepared in the same manner asthe nickel hydroxide electrode D of the foregoing example except that88.5 parts by weight of the nickel hydroxide powder covered by a cobaltcompound having a valence of greater than 2 and 11.5 parts by weight ofa nickel oxyhydroxide powder were used. The nickel hydroxide electrode Ewas then used to obtain a comparative battery E. The nickel hydroxideelectrode E thus obtained was then examined for average nickel valence.As a result, the nickel hydroxide electrode E was found to have anaverage nickel valence of 2.15.

Comparative Example 4

10 parts by weight of a cobalt hydroxide powder, 90 parts by weight of anickel hydroxide powder and 2 parts by weight of a carboxymethylcellulose powder were then added to purified water to prepare a slurrysample. A foamed nickel substrate was impregnated with the slurrysample, pressed, and then dried to obtain a nickel hydroxide electrodeF. A comparative battery F was then prepared in the same manner as inthe foregoing example except for the foregoing conditions. The nickelhydroxide electrode F thus obtained was then examined for average nickelvalence. As a result, the nickel hydroxide electrode A was found to havean average nickel valence of 2.00.

All these batteries had a positive electrode having a theoreticalcapacity of 1,000 mAh.

These batteries were each subjected to 10 cycles of 12 hour charging at100 mA and discharging to 0.8 V at 100 mA at room temperature so thatthe hydrogen-occluding alloy negative electrode was thoroughlyactivated, and then measured for discharge capacity under the followingconditions:

Charging . . . 120% (6 hours) at 200 mA

Discharging . . . To 0.8 V at 200 mA

Temperature . . . 25° C.

The results of test are shown in FIG. 2. The battery D of the presentinvention, which uses a nickel hydroxide powder covered by a cobaltcompound having a valence of 2 and a nickel oxyhydroxide powder coveredby a cobalt compound having a valence of greater than 2, exhibited adischarge capacity as great as about 1,030 mAh. On the other hand, thecomparative battery E, which comprises a nickel hydroxide powder coveredby a cobalt compound having a valence of greater than 2 and a nickeloxyhydroxide powder, and the comparative battery F, which uses a nickelhydroxide powder and a cobalt hydroxide powder, exhibited a dischargecapacity of about 1,000 mAh and about 960 mAh, respectively.

The reason why the battery D of the present invention exhibits a greaterdischarge capacity than the comparative battery F is because the batteryD of the present invention uses a nickel hydroxide powder and nickeloxyhydroxide powder covered by a cobalt oxyhydroxide having a highelectrical conductivity, which exhibits a higher electrical conductivitythan nickel hydroxide, present in the electrode plate. On the otherhand, the comparative battery E uses cobalt hydroxide incorporatedtherein as a starting material of cobalt oxyhydroxide used as anelectrically conducting agent merely in the form of powder. Further, thecomparative battery F uses less electrically conducting agent (cobaltoxyhydroxide) which maintains its electrical conductivity at the laststage of discharging in the vicinity of the nickel oxyhydroxide powderincorporated therein. This is presumably why the comparative battery Eexhibits a smaller discharge capacity than the battery of the presentinvention.

As mentioned above, the battery D of the present invention can maintainits high electrical conductivity at any stage during itscharge-discharge process, making it possible to realize a high percentutilization of active material.

Even if a small amount of a hydroxide such as zinc, cobalt and cadmiumis incorporated in a nickel hydroxide powder or a nickel hydroxidepowder having a valence of greater than 2, the properties of theelectrode of the present invention cannot be deteriorated. The oxidationof the nickel hydroxide powder for the production of a nickel hydroxidepowder having a valence of greater than 2 or the production of a cobaltcompound having a valence of greater than 2 can be accomplishedchemically or electrochemically.

Further, cobalt oxyhydroxide which has previously been coated on thesurface of an active material mainly composed of highly oxidized nickelhydroxide and an active material mainly composed of nickel hydroxide canbe applied to a nickel hydroxide electrode prior to the preparation ofalkaline storage battery to reduce the amount of an negative activematerial which cannot discharge electricity, making it possible toprovide the alkaline storage battery with an enhanced energy density.

What is claimed is:
 1. An electrode comprising: a positive activematerial comprising nickel hydroxide powder having a nickel valence ofgreater than 2, and a cobalt compound having a cobalt valence of greaterthan 2, wherein said cobalt compound is formed on the surface of saidnickel hydroxide powder.
 2. The electrode according to claim 1, whereinsaid positive active material comprising a first nickel hydroxide powderhaving a nickel valence of not greater than 2 with a cobalt compoundhaving a cobalt valence of greater than 2 formed on the surface thereofand a second nickel hydroxide powder having a nickel valence of greaterthan 2 with a cobalt compound having a cobalt valence of greater than 2formed on the surface thereof; and further wherein the average nickelvalence of said first and second nickel hydroxide powder is not lessthan 2.07.
 3. The electrode according to claim 1, wherein the averagenickel valence of said nickel hydroxide powder is less than the averagecobalt valence of said cobalt compound.
 4. The electrode according toclaim 1, wherein said three-dimensional porous material is athree-dimensional metal porous material.